Shiny, impact-resistant polystyrol which is resistant to tension cracks

ABSTRACT

A process for increasing the stress-cracking resistance of polystyrene molding compositions, stress-cracking-resistant, glossy polystyrene molding compositions, composite layered sheets or composite layered films with an outer layer of this type, and also processes for producing moldings for refrigeration equipment.

The invention relates to a process for increasing the stress-crackingresistance of polystyrene molding compositions, tostress-cracking-resistant, glossy polystyrene molding compositions, tocomposite layered sheets or composite layered films with an outer layerof this type, and also to processes for producing moldings forrefrigeration equipment.

Nowadays, internal containers and inner door panels in refrigerators andfreezers are mainly produced by thermoforming extruded semifinishedsheeting (sheets or films) made from specific impact-resistantpolystyrenes (ESCR-HIPS) which are stress-cracking-resistant withrespect to polyurethane blowing agents, such as cyclopentane.Stress-cracking-resistant impact-resistant polystyrenes generally have avery matt surface. This disadvantage can be overcome by coextruding agloss layer or by film-lamination.

Nowadays, gloss-layered coextrusion onto ESCR-HIPS mainly usesimpact-resistant polystyrenes with high surface gloss, also oftenblended with free-flowing standard polystyrenes (GPPS). A disadvantagehere is the marked impairment of stress-cracking resistance with respectto oils, fats, and cleaning compositions. This increases susceptibilityto damage by stress cracking, in particular in the case of internalcontainers for refrigerators or freezers.

Polystyrene molding compositions made from polystyrene and from athermoplastically styrene-based elastomer (STPE) are known, and examplesof descriptions of these are found in WO 96/20248 and WO 99/46330, wherethe STPE is mainly used to increase toughness or adjust mechanicalproperties. OZ WO 00/24578 describes incorporation of lubricant in theform of a masterbatch comprising styrene-butadiene block copolymer intocomposite sheets with polystyrene outer layers. The elastomericproperties and the transparency required mean that a STPE is present asmain component in the films described in WO 96/23823 and WO 97/46608.

It is an object of the present invention, therefore, to overcome theabovementioned disadvantages and to provide a process for increasing thestress-cracking resistance of polystyrene molding compositions, and alsoto provide polystyrene molding compositions which are bothstress-cracking-resistant and glossy.

We have found that this object is achieved by means of a process forincreasing the stress-cracking resistance of polystyrene moldingcompositions, where a styrene-based thermoplastic elastomer (STPE) isadmixed with the polystyrene molding compositions.

The STPE preferably has a tensile strain at break of more than 300%,particularly preferably more than 500%, in particular more than 600%,measured to ISO 527, and the amount admixed is in the range from 1 to40% by weight, preferably from 5 to 30% by weight, based on thepolystyrene molding composition. The STPE particularly preferablyadmixed is a linear or star-type styrene-butadiene block copolymer withexternal polystyrene blocks S and, between these, styrene-butadienecopolymer blocks with random styrene/butadiene distribution(S/B)_(random), or with a styrene gradient (S/B)_(taper).

The total butadiene content is preferably in the range from 15 to 50% byweight, particularly preferably in the range from 25 to 40% by weight,and the total styrene content is therefore preferably in the range from50 to 85% by weight, particularly preferably in the range from 60 to 75%by weight.

The styrene-butadiene block (S/B) is preferably composed of from 30 to75% by weight of styrene and from 25 to 70% by weight of butadiene. Theblock (S/B) particularly preferably has a butadiene content of from 35to 70% by weight and a styrene content of from 30 to 65% by weight.

The proportion of the polystyrene blocks S is preferably in the rangefrom 5 to 40% by weight, in particular in the range from 25 to 35% byweight, based on the entire block copolymer. The proportion of thecopolymer blocks S/B is preferably in the range from 60 to 95% byweight, in particular in the range from 65 to 75% by weight.

Particular preference is given to linear styrene-butadiene blockcopolymers of the general structure S-(S/B)-S having one or more,situated between the two S blocks, blocks (S/B)_(random) having a randomstyrene/butadiene distribution. Block copolymers of this type areobtainable by anionic polymerization in a non-polar solvent withaddition of a polar cosolvent or of a potassium salt, for example asdescribed in WO 95/35335 or WO 97/40079.

Suitable polystyrene molding compositions are standard polystyrene(GPPS), impact-modified polystyrene (HIPS), and polymer mixtures whichcomprise these polystyrenes, in particular glossy, impact-modifiedpolystyrenes (HIPS) with a surface gloss of at least 40%, preferably atleast 70%, in particular at least 80%, measured at an angle of 60° toDIN 67530, or a mixture of these with standard polystyrene (GPPS).

Depending on the nature of the STPE admixed, even small amounts canincrease the stress-cracking resistance of the polystyrene moldingcomposition. Preferred polystyrene molding compositions comprise

-   A) from 70 to 95% by weight, preferably 75 to 85% by weight, of a    glossy impact-modified polystyrene (HIPS) with a surface gloss of    more than 40%, preferably more than 70%, and particularly preferably    more than 80%, measured at an angle of 60° to DIN 67530, or a    mixture thereof with standard polystyrene (GPPS), and-   B) from 5 to 30% by weight, preferably from 15 to 25% by weight, of    a styrene-based thermoplastic elastomer (STPE).

The abovementioned styrene-butadiene block copolymers are suitable asSTPE.

The polystyrene molding compositions of the invention may moreovercomprise the usual auxiliaries and additives, such as lubricants,antiblocking agents, release agents, stabilizers, antistats, flameretardants, colorants, etc.

In another embodiment of the invention, the stress-cracking-resistant,glossy polystyrene molding compositions may be used as outer gloss layerin composite sheets or composite films. Preferred composite layeredsheets or composite layered films comprise a substrate layer made froman amorphous styrene polymer, such as standard polystyrene (GPPS) orimpact-modified polystyrene (HIPS), and comprise an outer layer madefrom the stress-cracking-resistant and glossy polystyrene moldingcomposition described above.

The substrate layer preferably comprises a stress-cracking-resistant,impact-modified polystyrene, for example where the composite layeredsheet is intended for producing moldings for refrigeration equipment.

The substrate layer is particularly preferably composed of from 20 to100% by weight of impact-resistant polystyrene (HIPS) and from 0 to 80%by weight of standard polystyrene. It is advantageous to useimpact-resistant polystyrenes with increased stress-cracking resistance,these being known as ESCR-HIPS. These generally have a gel content of atleast 30% and an average particle size of the disperse rubber phase,determined by light scattering, of at least 5 μm.

The outer layer generally has a thickness of from 1 to 10%, preferablyfrom 2 to 5%, based on the total thickness of the composite layeredsheet or composite layered film. The total thickness depends on theapplication, and may generally be from 0.1 to 10 mm, preferably from 0.5to 5 mm.

The outer layer preferably comprises from 0.1 to 5% by weight, based onthe polystyrene molding composition, of a lubricant, such as stearates.

The increased stress-cracking resistance of the polystyrene moldingcompositions of the invention, and also of the composite layered sheetsor composite layered films in which these are present as outer layer,together with their high surface gloss, make them suitable for producingmoldings for refrigeration equipment.

EXAMPLES Example PS 1

A styrene-butadiene block copolymer (STPE) as in WO 95/35335 (Styroflex®6105) and a free-flowing impact-modified polystyrene (BASF polystyrene454 C), each in the form of pellets, were mixed in a Rhönrad mixer andthen injection molded to give tensile test specimens to ISO 3167.

Example PS 2

Example 1 was repeated, except that the impact-modified polystyrene usedwas (BASF polystyrene 473 D).

Stress-cracking resistance (ESCR) was assessed using the tensile strainat break, determined by a method based on the “flex-strip method”described in ISO 4599. To this end, the injection-molded tensile testspecimens were clamped over flex templates with a radius of 170 mm andexposed for 10 and, respectively, 50 minutes to a cyclopentaneatmosphere (CP) in an autoclave at a pressure of 300 mbar, and thentensile-tested to ISO 527. Tensile test specimens with no exposure tosolvent (air) were tested as a reference.

The makeup of the specimens and results from Examples 1 and 2 are givenin Table 1.

TABLE 1 Stress-cracking resistance of injection-molded specimens as afunction of cyclopentane exposure time, measured in terms of tensilestrain at break [%] STPE [% by Example weight] 0 min 10 min 50 min PS 10 28 12 27 PS 1 5 36 35 45 PS 1 10 47 39 59 PS 1 15 63 74 63 PS 1 20 7487 85 PS 2 0 41 20 27 PS 2 5 44 34 50 PS 2 10 39 40 57 PS 2 15 48 55 85PS 2 20 71 105 116Composite layered sheets with gloss outer layer:

Examples E1 to E7 and Comparative Experiments C1 and C2

Composite layered sheets with a substrate layer (A) of thickness 2.9 mmand an outer layer (B) of thickness 0.1 mm, and the makeup listed inTable 2 were produced by coextrusion.

Products for Substrate Layer A:

(A 1): Impact-resistant polystyrene with a melt volume rate MVR 200/5 of3.4, a tensile modulus of elasticity of 1600 MPa, and a yield strain of21 MPa (polystyrene 2710 from BASF AG).

(A 2): Impact-resistant polystyrene with a melt volume rate MVR 200/5 of6.5, a tensile modulus of elasticity of 1400 MPa, and a yield strain of17 MPa (polystyrene 2712 from BASF AG).

(A 3): Mixture made from an impact-resistant polystyrene with a meltvolume rate MVR 200/5 of 4, a tensile modulus of elasticity of 1800 MPa,and a yield strain of 24 MPa (polystyrene 486 M from BASF AG) and astandard polystyrene with a melt volume rate MVR 200/5 of 3, a tensilemodulus of elasticity of 3300 MPa and a Vicat B temperature of 101° C.(polystyrene 158 K from BASF AG); blending ratio 80:20.

Products for Outer Layer:

High-gloss impact-modified polystyrene with a melt volume rate MVR 200/5of 4, a tensile-modulus of elasticity of 1900 MPa, and a yield strain of32 MPa (polystyrene 585 K, BASF)

High-gloss impact-modified polystyrene (polystyrene 555 G, BASF)

Free-flowing standard polystyrene with a melt volume rate MVR 200/5 of10, a tensile modulus of elasticity of 3300 MPa, and a yield strain of46 MPa (polystyrene 143 E, BASF)

Styrene-butadiene block copolymer as in WO 95/35335 (Styroflex® BX 6105,BASF) with a tensile strain at break to DIN 53455 of more than 650%(STPE).

TABLE 2 Makeup of layers of composite sheets: Makeup of outer layer (B);Experiment (A) (Data are proportions by weight) C 1 A 1 PS 585 K C 2 A 1PS 585 K:143 E (2:1) E 1 A 2 PS 585 K:Styroflex BX 6105 (97:3) E 2 A 1PS 585 K:Styroflex BX 6105 (95:5) E 3 A 1 PS 585 K:Styroflex BX 6105(90:10) E 5 A 2 PS 585 K:Styroflex BX 6105 (80:20) E 5 A 3 PS 585K:Styroflex BX 6105 (60:40) E 6 A 1 PS 555 G:PS 143 E:Styroflex BX 6105(50:30:20) E 7 A 1 PS 585 K:PS 143 E:Styroflex BX 6105 (50:35:15)

Examples E1 to E4, E6, and E7, and comparative experiments C1 and C2have very high surface gloss of from 85 to 95% for the outer layer,measured at an angle of 60° (DIN 67530). Surface gloss for example E5was 75%.

Stress-cracking resistance (ESCR) was assessed using the tensile strainat break, determined by a method based on the “flex-strip method”described in ISO 4599. To this end, tensile test specimens were producedfrom the composite panels, longitudinally and perpendicularly to thedirection of extrusion, and were clamped over flex templates with radiusof r1=1000 mm and r2=500 mm. A mixture made from olive oil/oleic acid(1:1) was used to coat the resultant gloss layer upper sides undertensile stress. After aging for 24 h, the test specimens were subjectedto the ISO 527 tensile test. Clamped tensile test specimens with noexposure to solvent (air) were tested as a reference (indicated as 0specimen below).

The results from the stress-cracking studies show that conventionalouter layers as in comparative experiments C1 and C2 are highlysusceptible to stress cracking with respect to the test medium of oliveoil/oleic acid (OO/OA). Most of the specimens break within the contacttime of 24 hours. Examples E1 to E7 show that the tensile strains atbreak rise markedly as STPE content increases, the outer layers beingsubstantially more stress-cracking resistant.

At 40% Styroflex modification (Example E5) very high stress-crackingresistance (ESCR) is obtained. However, the gloss and scratch resistance(Table 4) of the outer layer do not achieve the level obtained withsmaller amounts of Styroflex modification.

Results of Scratch Resistance Studies:

The gloss layer sides were tested using a scratch tester and 3 types ofscratch: F=0.7 N/1.4 N/2.6 N (diamond tip angle =120°, radius=0.2 mm)

TABLE 4 scratch resistance and scratch depth [μm] Scratch depth (μm) at:composite layered sheet: F = 0.7 N F = 1.4 N F = 2.6 N C1 1.0 4.9 12 C20.9 4.8 9 E1 1.2 5 8 E2 1.3 5.2 7 E3 1.2 5.3 7.5 E4 1.6 6.1 12 E5 3.8 1022 E6 1.1 5.8 11 E7 1.0 4.1 9.7

TABLE 2 Results from stress cracking tests: Flexing radius [mm] C1 C2 E1E2 E3 E4 E5 E6 E7 TB l/0 specimen 1000 35 28 34 36 39 48 56 36 34 TB t/0specimen 1000 32 26 30 32 36 45 48 32 30 TB l/24 hrs OO/OA 1000  4  3  512 23 35 45 28 26 TB t/24 hrs OO/OA 1000 br. br. br.  4 18 27 42 18 20TB l/24 hrs OO/OA  500  2 br.  3  8 21 28 43 25 20 TB t/24 hrs OO/OA 500 br. br. br.  2 14 25 32 12 10 TB = tensile strain at break in (%);l = longitudinal; t = transverse br.: tensile test specimens fracturedduring the experiment

1. A composite layered sheet or composite layered film comprising asubstrate layer and an outer layer, wherein the substrate layer is madefrom an amorphous styrene polymer and the outer layer is made from apolystyrene molding composition comprising A) from 70 to 95% by weightof a glossy impact-modified polystyrene (HIPS) which has a surface glossof more than 40%, measured at an angle of 60° according to DIN 67530, ora mixture thereof with standard polystyrene (GPPS), and B) from 5 to 30%by weight of a styrene-based thermoplastic elastomer (STPE).
 2. Thecomposite layered sheet or composite layered film of claim 1, whereinthe STPE has a tensile strain at break of more than 300%, measuredaccording to ISO
 527. 3. The composite layered sheet or compositelayered film of claim 1, wherein the STPE is a linear or star-typestyrene-butadiene block copolymer with external polyatyrene blocks Sand, between these, styrene-butadiene copolymer blocks with a randomstyrene/butadiene distribution (S/B)_(random) or with a styrene gradient(S/B)_(taper).
 4. The composite layered sheet or composite layered filmof claim 1, wherein the substrate layer comprises a stress-crackingresistant, impact-modified polystyrene.
 5. The composite layered sheetor composite layered film of claim 1, wherein the outer layer has athickness of from 1 to 10%, based on the total thickness of thecomposite layered sheet or composite layered film.
 6. The compositelayered sheet or composite layered film of claim 1, whose totalthickness is from 0.1 tO 10 mm.
 7. The composite layered sheet orcomposite layered film of claim 1, wherein the outer layer consistsessentially of from 15 to 25% by weight of the styrene-basedthermoplastic elastomer (STPE) and from 75 to 85% by weieht of theglossy impact-modified polystyrene (HIPS).
 8. The composite layeredsheet or composite layered film of claim 1, wherein the impact-modifiedpolystyrene (HIPS) has a surface gloss of more than 70%, measured at anangle of 60° to DIN
 67530. 9. The composite layered sheet or compositelayered film of claim 1, wherein the impact-modified polystyrene (HIPS)has a surface gloss of more than 80%, measured at an angle of 60° to DIN67530.
 10. A molding which is in form of refrigeration equipment andwhich comprises the composite layered sheet or composite layered film ofclaim
 1. 11. The composite layered sheet or composite layered film ofclaim 10, wherein the styrene polymer of the substrate layer comprisesan impact-modified polystyrene (HIPS) which has a gel content of atleast 30% and comprises a disperse rubber phase having an averageparticle size of at least 5 μm, determined by light scattering.
 12. Thecomposite layered sheet or composite layered film of claim 1, whereinthe styrene-based thermoplastic elastomer (STPE) is a styrerie-butadieneblock copolymer which comprises from 5 to 40% by weight, based on theweight of the block copolymer, of polystyrene blocks S.